1. Field of the Invention
The present invention relates to a hologram and more particularly to a hologram produced by using novel sensitive material, and further to a method of producing the hologram.
2. Description of the Prior Art
Holography is a unique technique of forming an optical image by which an object is irradiated by a well coherent wave such as laser where the wave is modulated in amplitude and phase in accordance with the shape of the object, the modulated wave reflected upon or transmitted through the object is recorded (=hologram) and the hologram is irradiated again by the laser so as to reproduce an optical image of the original object.
With the recent progress of study and research on holography, it has been made clear to a certain extent what material is suitable for use in holography or what characteristics such a hologram recording material should have.
Thus, there have already been proposed various materials such as bleached silver salt (U.S. Pat. No. 3,672,744), photoresist, thermoplastic resin, dichromate gelatin (U.S. Pat. No. 3,617,274), inorganic glass material and ferroelectric substance, and at present a further study on the characteristics of these materials is proceeding in the art.
As described above, holography is a unique and novel information recording system and therefore it necessitates particular hologram recording materials which are able to satisfy much more sever requirements than those of the conventional recording system. Although many attempts and efforts have been made to develope novel and improved materials useful for holography, there has not yet been proposed any recording material having good characteristics and properties enough to completely satisfy the severe requirements.
For example, it has been attempted to use the known silver salt emulsions for the conventional photography also as recording materials for holography, making full use of their excellent properties. However, photographic silver salt emulsion has some important disadvantages. When it is employed to form a hologram, then an information pattern has to be recorded by the density distribution of silver grains through the processes of developing and fixing. The hologram thus obtained is an amplitude modulation type hologram which has a demerit of substantial light loss in reproducing the image.
In the conventional photography, an image of object can be recorded directly. But, in case of holography, the image must be recorded as a very complicate interference pattern and thereby a high resolving power is required. The grain structure inherent in photographic photosensitive emulsion adversely affects for this purpose. For the reason, the use of photographic emulsion in holography is limited.
It is known that a phase modulation type hologram based on not the change in light absorption but the change in refractive index through a recording medium allows to give a high degree of diffraction with less loss in light, compared with the above mentioned amplitude modulation type hologram.
In order to produce such phase type hologram, hitherto have been used bleached type photographic photosensitive emulsion and dichromate gelatin.
It is said that holograms produced by using dichromate gelatin are superior to those produced by using bleached type photographic emulsion in respect to diffraction efficiency, S/N ratio and other properties. However, dichromate gelatin does not allow the production of holograms having adequate stability which is one of the most important properties required for hologram recording materials. In particular, its moisture resisting property is so poor that the recorded hologram may be lost by the action of moisture. This is a critical disadvantage of the material.